Fan is controlled by the car DME. It turns the fan on and control the speed of the fan. You can command the fan to turn on if you have the correct BMW software. The software will also tell you if you have a faulty fan.
Never heard of the lighting test, not unless he is using a test light to show you that you don't have power to the fan. In that case yes the fan is no good and the fuse to the fan will also need to be replaced.
A fan can be brought for anywhere between 400 and 600 dollars (U.S.) . It a 3 - 4 hours labor to remove the old fan and install a new fan.
See this post on aux fan.
http://www.xoutpost.com/bmw-sav-foru...fan-fault.html